TUD Java Course Lesson 1

https://github.com/Trivo25/tud-java-course

October 26, 2021

While you’re waiting for it to start…

  • We’re checking your 3G status
  • Check your E-Mail if you haven’t done so yet
    • setup WiFi
    • install VS Code & OpenJDK
    • check out this course’s page https://trivo25.github.io/tud-java-course/

First: Which Language?

Anyone in here who needs us to speak English? Else, we’ll hold it entirely in German.

Java

Object Oriented Programming

Florian Kluge, Moritz Schulz 21. Oktober 2021


Florian.Kluge@mailbox.tu-dresden.de

Moritz.Schulz2@mailbox.tu-dresden.de

Overview

  1. General Info about this course
  2. Check Set-Up
  3. Start coding!
    • learn about variables & user input

Organization

About us

Florian Kluge

Florian.Kluge@mailbox.tu-dresden.de

Moritz Schulz

Moritz.Schulz2@mailbox.tu-dresden.de

What are we doing here?

  • Introduction to programming
  • Getting to know the basics of Java
  • Preparation for upcoming courses (e.g. “Softwaretechnologie”, 2nd Semester)

Structure

  • 14 lessons
  • every Thursday, 13:00 - 14:30
  • APB/006 (right here)
  • 3G-Regel (vaccinated / recovered / tested)
  • Attendance list

Attendance

  • This course is held on a voluntary basis.
  • You’re here voluntarily.
  • If you want to quit, please let us know so we can invite students from the waiting list.
  • If you don’t attend the course for 2 weeks in a row without notice
    • we will give your slot to other students.

Our course philosophy

  • This course is centered around you.
  • Coding is best learned by doing it.
  • Illustrative examples help.
  • Mistakes are good because they help us learn.
  • We’re not flawless experts either.
  • Please ask questions
    • because in the end, it’s about your understanding.
  • We’ll walk through the class room to check that everyone gets along.

Why Java?

  • widely used & modern programming language
  • helpful ways of structuring code
  • can be used for lots of things
  • the same program can run on most computers
  • good for getting started

Pop Quiz Time

Do you have any programming experience already?


We’re about to get started…

  • we need Java OpenJDK 11
    • https://adoptium.net
  • check if it’s installed properly:
    • open a terminal
      • Windows: Windows+R => cmd => Enter
      • MacBook: ⌘ + T
      • Linux (depends): Ctrl+Shift+T
    • enter: javac -version
    • it should say: javac 11.0.12

Doesn’t work?

Use an online compiler for now.

https://www.jdoodle.com/online-java-compiler/

Let’s go!

  • skip these steps if you’re using the online IDE
  • in VS Code:
    • on the right side, select a new folder Programmierkurs_Java_1
    • create a new file HelloWorld.java in that folder
    • open the file

Your first piece of code

Now it’s time to write your first piece of code!

public class HelloWorld {
    public static void main (String[] args ) {
        System.out.println("Hello World!");
    }
}

File: HelloWorld.java

Pay attention to copy the code exactly, keeping spaces, semicolons, parentheses, curly braces, capitalization, … exactly as above.

Run the program

  • save the file: File > Save
  • For VS Code users:
    • open the terminal: View > Terminal
    • type: javac HelloWorld.java
    • type: java HelloWorld
    • see: Hello World!
  • For the online users:
    • click: [Run!]
    • see: Hello World!

Let’s play around a bit

  • change the text
  • try to run the program
    • … (like we did before)

Let’s explain… (1/4)

  • Coding (= Programmieren) is telling the computer what to do.
  1. Coding = We list instructions for the computer.
    • precise
    • step by step
  2. A program called compiler translates code so the computer can understand it.
  3. The computer runs the program.

Let’s explain… (2/4)

  1. We write code that humans can read.
    • HelloWorld.java
    • let’s look at the code again (next slide)
  2. The compiler javac translates the code so the computer understands it.
    • HelloWorld.java => HelloWorld.class
  3. The computer runs the program.
    • command: java HelloWorld

Let’s explain… (3/4)

This is the framework of every Java program:

  • HelloWorld is the class name and should be like the file name, but without .java
  • start inside public static void main (String[] args) { … }
public class HelloWorld {
    public static void main (String[] args) {
        
    }
}

Let’s explain… (4/4)

This is the piece of code

that prints Hello World!

System.out.println("Hello World!");

Introducing: Variables

public class HelloWorld {
    public static void main (String[] args) {
        String phrase = "Hello World!";
        System.out.println(phrase);
    }
}

About Variables

String phrase = "Hello World!";
  • they have a type: this one is a String (basically a piece of text)
  • they have a name: this one is called phrase
  • they can be created (formally: declared): =
  • they have a value: "Hello World!"
    • note the "": they tell Java that this is text, not code
  • think of them like a box that can only store things of a specific type

What do we need variables for?

  • we can store data in them
  • we can re-use them
    • avoid typing their values twice
public class HelloWorld {
    public static void main (String[] args) {
        String phrase = "Hello World!";
        System.out.println(phrase);
        System.out.println(phrase);
    }
}

Strings can be merged (concatenated)

public class HelloWorld {
    public static void main (String[] args) {
        String greeting = "Hello";
        String name = "World"
        System.out.println(greeting + " " + name + "!");
    }
}

prints: Hello World! (just as before)

Let’s talk to our program!

Remember to copy the first line as well!

import java.util.Scanner;
public class Talk {
    public static void main (String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("Hi, what's your name?");
        String name = scanner.nextLine();
        System.out.println("Hello " + name + "!");
    }
}

file: Talk.java

  • online coding: turn “Interactive” on
  • remember to press Enter after entering your name

By the way, we can use comments.

// I am a comment. I can explain things.
  • comments are ignored by Java
  • we can use them to explain our code (to ourselves)
  • next, I’ll use comments to explain the previous code

Let’s explain #2… (1/2)

// use somebody else's code, so we don't need to
// tell the computer how exactly to read input 
import java.util.Scanner;

// same framework as before:
public class Talk {
    public static void main (String[] args) {
        // create a new variable of type Scanner
        // that reads from the console (System.in)
        Scanner scanner = new Scanner(System.in);
        
        // Ask the user about their name:
        System.out.println("Hi, what's your name?");
        //...

Let’s explain #2… (2/2)

        //...
        
        // Read what the user wrote,
        // and save it in the variable called "name"
        String name = scanner.nextLine();
        
        // Using the name, greet the user!
        System.out.println("Hello " + name + "!");
    }
}

Java also knows numbers

int answer = 42;
  • answer is a variable of type int
  • type int (integer) stores whole numbers
    • like 7, 78482, -420

We can also read numbers

import java.util.Scanner;
public class TalkAgain {
    public static void main (String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("Hi, how old are you?");
        int age = scanner.nextInt();
        int age2 = age + 5;
        System.out.println("In 5 years, you'll be " + age2);
    }
}

file: TalkAgain.java

Adding numbers works!

int num = 42 + 17;
int num2 = num + 7;
  • it doesn’t matter if it’s the number itself or a variable containing a number
  • some operators on numbers: +, -, *, /
  • notice that an int divided by an int will still be an int
    • we’ll learn about floating point numbers soon

What have we learned?

  • how to print text to console
  • how to declare variables of type int, String
  • how to read input from the console
  • that operators like +, -, * and / exist

Apply your new-learned knowledge

  • Let’s build a calculator!
  • Suggestion on how to do that:
    • read one number
    • save it in a variable
    • read and save another number
    • add them
    • print the result

That’s it!

  • Be encouraged to keep working on the calculator task :)
  • Feel free to reach out to us
    • to send us your results
    • to tell us about problems you ran into

Next lesson

  • a few more types of variables
  • control flow: if-statements, while-loops
  • more practical examples!

See course materials here: https://trivo25.github.io/tud-java-course/